Vertical orientation device

ABSTRACT

&#39;&#39;&#39;&#39;Gravity-gradient&#39;&#39;&#39;&#39; rod is connected to satellite by powered angular drive means controlled by attitude sensors which displace &#39;&#39;&#39;&#39;gravity-gradient&#39;&#39;&#39;&#39; rod in proper direction to produce increased torque by interaction with gravitational field to maintain satellite in desired attitude toward local gravitational gradient.

United States Patent Wanger 5] Feb. 8, 1972 [54] VERTICAL ORIENTATIONDEVICE [72] Inventor: Robert P. Wanger, Valley Forge, Pa.

[ 73] Assignee: General Electric Company [22] Filed: July 13, 1967 21Appl. No.: 653,195

OTHER PUBLICATIONS Hutchcr. Norman M.: Spacecraft Attitude Sensors-Where We Stand Today: Astronautics and Aeronautics; Dec. I966. pp.58-59.63.

Roberson. Robt. E.; Attitude Control; Advances in Space Science. Vol. 2.1960; pp. 4l5-4l9 Primary Examiner-Milton Buchler AssistantExaminer-Jeffrey L. Forman Attorney-Paul F. Prestia, Allen B. Amgott,Henry W. Kaufmann, Melvin M. Goldenberg, Frank L. Neuhauser and Oscar B.Waddell [57] ABSTRACT Gravity-gradient rod is connected to satellite bypowered angular drive means controlled by attitude sensors whichdisplace gravity-gradient rod in proper direction to produce increasedtorque by interaction with gravitational field to maintain satellite indesired attitude toward local gravitational gradient.

4Claims,2Drawingfigures PATENTEDFEB 8 m2 3. 640.487

uvvnvron: Roms/er f. WA NGER,

W M aw VERTICAL ORIENTATION DEVICE This invention pertains to the art ofstabilizing space satellites with respect to the local vertical. It isknown in the prior art to stabilize a space vehicle with respect to thelocal vertical by making the moments of inertia around each of twoorthogonal axes of the vehicle appreciably larger than that around athird axis orthogonal thereto. This causes the vehicle, in agravitational field, to tend to assume an attitude such that the thirdaxis is aligned with the local vertical, i.e., to the vector gradient ofgravity, whose direction defines the local vertical. This is commonlyachieved by extending from the vehicle a long rodlike structure, longerthan any dimension of the vehicle, which, despite small mass, may havelarge moment of inertia because of large radius of gyration, and whichmay be provided with a damping device to damp oscillations of thevehicle around its preferred position. While such damping devices arehighly satisfactory in damping oscillations, and vehicles thusstabilized show great stability, the gravitygradient stabilizationsystem has the characteristic of most self-nulling passive systems thatthe torque tending to restore the vehicle to its desired attitudedecreases continuously as the vehicle approaches that attitude. Becausevarious periodic and random torques tend to displace from its desiredattitude a satellite in orbit around the earth, this smallness of torquewhen the vehicle is nearly but not quite at its desired attitude keepsthe vehicle from following the angulm variations in the local verticalso rapidly and hence so accurately as would be desirable. In consequenceof this, a vehicle stabilized in attitude solely by gravity-gradientmeans, however well damped, shows displacements from its desiredattitude to the local vertical which are great enough to be detected byattitude sensors of the known art, such as infrared sensors. Also, thelong, slender rodlike structures commonly used for gravity-gradientstabilization tend to bend under nonuniform heating from one-sidedexposure to the sun, so that the actual direction of the center of massof the rod from its point of attachment is not that which would bededuced from the orientation of the rod end at its point of attachmentto the vehicle. This, obviously, will produce a systematic error in asystem which relies for its functioning upon the validity of such adeduction.

It is an object of my invention to provide gravity-gradientstabilization means which provide accuracy of attitude control to thelocal vertical which is greater than that obtainable by purely passivemeans, and which employs attitude errors detected by attitude sensors tocorrect such errors.

To this end I provide otherwise conventional gravitygradientstabilization means with power-driven torque means to alter the angularposition (that is, tilt) of the gravitygradient means with respect tothe vehicle, responsively to attitude errors sensed by sensing means. Inone embodiment of my invention, a two-axis gimbal drive is so connectedto attitude sensing means that, when the vehicle tilts toward a givenazimuthal direction, an extended gravity-gradient rod is caused to tilt,relative to the vehicle, in the opposite direction. In a conventionalgravity-gradient stabilized vehicle, with fixedly attached rod, thetilting of the vehicle will cause a tilt of the rodthrough an equalangle, and will thus produced a correcting torque proportional, forsmall angles, to the magnitude of the vehicles tilt, or axial angulardisplacement from the vertical. But my invention causes the rod to moveby an angle which is a multiple greater than one of the tilt angle, andthus produces, by interaction with the local gravitational field, atorque which is proportionately greater than that producible by a purelypassive system.

The movement of the rod may be made to depend exclusively upon thesensed deviations from desired attitude, in closed-loop servo fashion,without dependence upon any a priori assumption that the direction inwhich the rod points at its connection to the vehicle is the directionto its center of mass. Consequently it is not necessary to predict anythermal or other bending.

Since the moment of inertia of the rod may, because of its length, becomparable with that of the more massive but more compact vehicle, theattitude of the vehicle may be corrected as rapidly as the power of therotational means permits by rotating the rod through an angle sufficientto rotate the vehicle to its proper attitude. In view of Newton's thirdlaw, the angles through which the vehicle and the rod rotate will beopposite in sign and inversely proportional in magnitude to theirrespective moments of inertia about their common point of rotation.Thus, given an initial vehicle tilt, the rapid corrective action mayleave the vehicle itself correctly oriented, but its rod out of linewith the local vertical. Since the combination of vehicle and rod willordinarily be designed so that the vehicle is stable in its desiredorientation when the rod is aligned with the local vertical, thisdeviation of the rod from alignment with the local vertical will leavethe vehicle in a condition which is unstable, in that the gravitygradient will act upon the vehiclerod combination so as to tilt thevehicle, if the rod angle with respect to the vehicle remains unchanged.It is possible, given the parameters of the vehicle and the rod, tocalculate the rate at which the rod should be rotated back intoalignment with the vertical to restore it to a stable position withouttilting the vehicle; and to provide a control computer to control thepowered rotating means to rotate the rod at this rate. However, thiswould require either that the rod be free from bending or that somemeans (such as thermocouples located to sense the temperatures atvarious portions of the rod) be provided to determine the rod bendingand provide a correcting input to the computer. While this proceduremight be desirable in special situations, it is not worth whileordinarily for the practical reason that the accuracy with which thevehicle is oriented, in the practice of my invention, will be determinedby the accuracy of the attitude sensors. There is, therefore, little tobe gained by providing special means to preserve an attitude which maybe in error by any error inherent in the sensors. It is simpler, andusually quite as satisfactory, to allow the rod to remain out of linewith the local vertical until this misalignment causes the vehicle totilt by an amount detectable by the attitude sensors, in which case theywill cause the rod to move by an amount sufi'rcient to correct thedetected tilt. Thus, like any closed-loop servosystem, the system willapproach its final position by means of measurements of its ownperformance. Operation in this mode has the further advantage that itautomatically adapts itself to situations in which external forces (suchas solar radiation pressure, or the action of the ambient magneticfield) tend to tilt the vehicle so that the rod-vehicle system isactually stable with the rod out of line with the local vertical.

For the better understanding and explanation of my invention I haveprovided figures of drawing, in which FIG. 1 represents schematicallythe elements of an embodiment of my invention in a vehicle in orbitaround the earth, and

FIG. 2 represents schematically a torque gimbal suitable for use in anembodiment of my invention.

Referring to FIG. 1, there is represented a vehicle body 10 in orbitabove the partially represented earth l2. Schematically represented byconical hoods are sensors 14 and 16, located diametrically opposite eachother, and sensors I8 and 20, located diametrically opposite each otheralong a line at right angles to that between sensors 14 and 16. Thesesensors may comprise therrnopiles or semiconductor devices sensitive toinfrared radiation from the earth, having associated with them opticalsystems so designed and oriented that, when the satellite is at theheight above the earth at which it is intended to operate, each sensorviews the horizon of the earth and a part of space above the earth.Since the earth radiates readily detectable amounts of infrared energy,while space does not, the total amount of energy received by each sensorat a given altitude will be dependent upon the tilt of the satellite,which determines what fraction of the sensor's field of view is filledby the earth, the remainder of the field of view being filled by space.Thus, when the satellite is properly oriented with respect to the earth,each of sensors l4, l6, l8, and 20 will these, and more sophisticatedversions thereof (cf. [15. Pat. No. 3,020,407) are well known in theart. They suffer from the disadvantage that when the satellite is insuch a position with respect to the earth and the sun that the sun isseen at or near the horizon, the sensor will give an erroneousindication. Vari ous means are known to prevent the sensors fromfunctioning under these conditions; for example, advantage may be takenof the fact that the output of a sensor viewing the sun will be muchgreater than that produced by viewing the earth alone, and circuitry maybe provided which will disconnect both the sensor viewing the earth andits diametric opposite so long as the excessively great output of one ofthem indicates that the sun is in view. Alternatively, a blind or shademay be caused by the excessively great signal or other suitabledetecting means to pass in front of the sensor and its opposite whilethe sun is in the field of view of either one. This will inactivate anysystem dependent upon the operation of the sensors; but the length oftime that the sun is in view of the sensor may be made sufficientlybrief, by suitably limiting the field of view of the sensor's opticalsystem, that the vehicle will not have sufficient time to deviate verymuch from its desired attitude, since both the disturbing torques andthe correcting torques are so small relative to the moments of inertiaof the vehicle that the period of oscillation of the system is ratherlong compared with the time envisaged. Still another alternative is toprovide additional sensors aimed at points on the horizon not occupiedby the sun, and to connect to these when the magnitude of signal from agiven sensor indicates that it is viewing the sun.

in the center of the vehicle body there is represented a twoaxis poweredtorquing device 22, which may conveniently be a bimetallic electricallyheated device. Such an embodiment includes two bimetallic spirals 24 and26 which are connected to a cross-shaft 28. Spirals 24 and 26 are soarranged that, when the temperature of both is changed by the sameamount, they will produce equal torques tending to drive shaft 28 isopposite directions, in consequence of which it will not turn. If,however, one such spiral is heated to a temperature higher than that ofthe other, as by passing an electric current through it, or through aheating winding in close proximity to it, the heated spiral will producea torque which will be greater than that produced by the other, unheatedspiral, and will turn shaft 28 until the torque of the unheated spiralis increased by the turning to a value equal to that of the heatedspiral. Such a thermal motor has the advantage that it does not requireslid ing bearings (which may be difiicult to lubricate in the cold andvacuum of space) and it is not subject to static friction, which tendsto produce somewhat jerky operation. While its operation is slowcompared with the speed of operation of conventional motors, it is amplyfast for the purposes of my invention. Also, by making the bimetalelements sufficiently large, they may be made of high torque without thenecessity of employing gear reductions or of using excessively highcurrents or any magnetic structure. The thermal motor is alsoparticularly well adapted to operation over a small angle. Thus, whileother conventional drive means may be employed in its stead, I preferthe thermal motor for my embodiment, as represented pictorially in FIG.2.

Connected to shaft 28, and at right angles to it, is another cross-shaft30, to which are connected bimetal spirals 32 and 34, similar to bimetalspirals 24 and 26 and, like them, arranged so that equal changes in thetemperature of both will cause them to produce equal and oppositelydirected torques. The ends of spirals 32 and 34 not connected to shaft30 are finnly fixed to plate 36 so that such equal and opposite torqueswill not permit them to rotate with respect to shaft 30, but unequalheating of spirals 32 and 34 will produce a net rotation. it is thusevident that, by suitable selective heating of appropriate ones ofspirals 24, 26, 32, and 34, plate 36 may be tilted around two orthogonalaxes identical with the axes of cross-shafts 28 and 30 and thus, withinthe constraints produced by mechanical interference, may tilt in anydirection.

Upon plate 36 there is mounted a motor-driven reel 38, which serves inaccordance with the known art to store, in rolled condition duringlaunching, a prestressed metal strip 40 which, after the vehicle 10 isplaced in orbit, is unrolled by operation of the motor of reel 38 toextend, being caused by its prestressed condition to form a tubularstructure which extends like a rod, and is usually referred to in thelanguage of the art as a gravity-gradient red. A damper 42 to damposcillations is usually conventionally attached to such a rod and ishere represented.

Thus far there have been described a system of sensors capable ofsensing the attitude of the vehicle 10 with respect to the earth'shorizons, and a powered torquing device 22 carrying a gravity-gradientrod which can alter the angle between the rod 40 and the vehicle body10. it remains to describe the electrical system which causes theindications of the sensors to control the angle of rod 40 to the vehiclebody 10 appropriately to the practice of my invention. It should benoted expressly, what is already implicit, that the control of vehicleattitude with respect to the earths horizons is a sufliciently goodapproximation to control with respect to the local vertical that the twoare regarded in the art as equivalent; the earth is suffi ciently nearto being a perfect sphere so that differences between the two are withinthe ordinary limits of accuracy of stabilization. A simple electricalsystem for carrying out my invention is comprised by four identicalamplifiers, which may be semiconductor devices, whose inputs are adaptedto receive the outputs of the sensors, and responsively thereto toproduce currents at voltages suitable to apply to heat the bimetalspirals to cause them to produce torques. As represented by rectangles(their design being well within the conventions of the known art),amplifier 44 has its input connected to sensor 14 and its output tospiral 24; amplifier 46 has its input connected to sensor 16 and itsoutput to spiral 26. Similarly, amplifier 48 is connected appropriatelybetween sensor 18 and spiral 32, and amplifier 50 between sensor M) andspiral 34. Since the direction of torque produced by heating abimetallic spiral depends upon the difference in coefiicients of thermalexpansion of the inner and outer metals forming the bimetals, whichdifference may be of either sign; and since amplifiers may be designedto produce either an increased or a reduced current output withincreasing input, the required relation may most simply be defined bythe statement that an increase in the infrared input to a sensor shouldtend to cause rod 40 to tilt in the direction of that sensor. Suchtilting will produce a reaction on the vehicle 10 body which will tendto cause the given sensor to raise its angle of view above the horizon,thus reducing the amount of infrared it receives from the earth. If twoopposed sensors, such as 14 and 16, receive equal amounts of infrared,indicating that vehicle 10 is properly oriented around the axis of shaft28, amplifiers 44 and 46 will receive equal inputs, will produce equaloutputs to similar but opposed bimetallic spirals 24 and 26, which willproduce equal and opposed torques, and will not rotate rod 40 around theaxis. Similarly equality of infrared inputs to sensors 18 and 20 willproduce equal and opposed torques on shaft 30, and rod 40 will not berotated around the axis of shaft 30. Any inequality in the irradiationof two opposed sensors, such as 14 and 16, or 18 and 20, will causeinequality in the torques applied to opposite ends of the appropriatecross-shaft, 28 or 30, and will result in a tilt of rod 40 in theappropriate direction to correct the unbalance.

To summarize, there have been disclosed attitude sensing means (l4, 16,18, 20) connected to control means (44, 46, 48, S0) to cause poweredtorque means (22) to adjust the relative angle between two parts (10,40) of a vehicle system responsively to the indications of the attitudesensing means l4, l6, 18, 20) to adjust to a predetermined attitude thepart 10) of the vehicle system whose attitude is sensed by the sensingmeans l4, l6, 18, 20).

More particularly, the powered torque means (22) may comprise opposingpairs (24 and 26, or 32 and 34) of torque devices, one member of a pairbeing separately responsive to the indication of one of a pair l4 and16, or 18 and 20) of oppositely directed sensors, whereby the netrotation produced by such a pair of opposed torque devices is determinedby the difference between the parameters sensed by the members of thepair of oppositely directed sensors.

In the particular embodiment disclosed, the sensors are infraredradiation sensors, and the torque devices are thermal motors. Further,the part 40 of the vehicle system is a rodlike structure adapted to bestored in a coiled or rolled condition, and is equipped with mechanicaloscillation damping means (42).

I claim:

1. ln a space vehicle comprising:

a vehicle body having a given mass and a gravity-gradient rod of massless than the said given mass,

connected at one of its ends to the said vehicle body, of length greaterthan any dimension of the vehicle body, extending away from the vehicle,and adapted to align itself with the local gradient of gravity,

the improvement comprising:

attitude sensing means to sense the attitude of the vehicle body withrespect to a reference;

powered torque means forming the connection between the said vehiclebody and the said one end of the gravitygradient rod, operativelyconnected to the said attitude sensing means to displace the saidgravity-gradient rod angularly with respect to the said vehicle bodyresponsively to and in the direction of an attitude deviation of thevehicle body with respect to the said reference sensed by the saidattitude sensing means.

2. The device claimed in claim 1 in which to said powered torque meanscomprises a pair of separately powered torque devices producing opposingtorques.

3. The device claimed in claim 2 in which each said opposed torquedevice is connected to be separately responsive to the indication of oneof a pair of oppositely directed sensors.

4. An orientation changing and momentum control apparatus for asatellite body comprising: an elongated rigid gravity-gradient rodhaving a pivotal connection with said satellite body, and torque motormeans for effecting relative angular movement between said elongatedrigid rod and said satellite body about said pivotal connection.

1. In a space vehicle comprising: a vehicle body having a given mass anda gravity-gradient rod of mass less than the said given mass, connectedat one of its ends to the said vehicle body, of length greater than anydimension of the vehicle body, extending away from the vehicle, andadapted to align itself with the local gradient of gravity, theimprovement comprising: attitude sensing means to sense the attitude ofthe vehicle body with respect to a reference; powered torque meansforming the connection between the said vehicle body and the said oneend of the gravity-gradient rod, operatively connected to the saidattitude sensing means to displace the said gravity-gradient rOdangularly with respect to the said vehicle body responsively to and inthe direction of an attitude deviation of the vehicle body with respectto the said reference sensed by the said attitude sensing means.
 2. Thedevice claimed in claim 1 in which the said powered torque meanscomprises a pair of separately powered torque devices producing opposingtorques.
 3. The device claimed in claim 2 in which each said opposedtorque device is connected to be separately responsive to the indicationof one of a pair of oppositely directed sensors.
 4. An orientationchanging and momentum control apparatus for a satellite body comprising:an elongated rigid gravity-gradient rod having a pivotal connection withsaid satellite body, and torque motor means for effecting relativeangular movement between said elongated rigid rod and said satellitebody about said pivotal connection.